1. Field of the Invention
The present invention relates to a motor control system, and more particularly to a motor control system for a three-phase brushless DC motor which is used, for example, as an assisting motor in an electric power steering system.
2. Description of the Related Art
Recently, a technique for assisting torque by using a motor instead of a conventional hydraulic power assisting system has been brought into practice for the purpose of reducing the vehicle weight and improving fuel economy.
For example, a three-phase brushless DC motor, which is a comparatively quick-responsive, can produce high torque, and is easy to control, is used as an assisting motor in an electric power steering (EPS) system. In the three-phase brushless DC motor, the produced torque is changed in proportion to a current. To accurately control the three-phase brushless DC motor, therefore, the current flowing through the motor requires to be detected with accuracy.
To that end, a method disclosed in, e.g., Patent Document 1 (JP-A-2004-117070) has been employed in the past. According to the disclosed method, a current sensing resistor (shunt resistor) is externally added, and a potential difference generated by a motor driving current across the current sensing resistor is amplified by an amplification circuit, e.g., an operational amplifier, and is taken into a microcomputer through the A/D conversion function. By using the taken-in data, current value feedback control is performed so that the current flowing through the current sensing resistor is matched with a target current. The motor is thus driven in accordance with PWM control.